Generate `baler-preload-map.json`

[DrewML]

For the Graph Preloading functionality, it's necessary for baler to spit out a mapping of JavaScript dependencies needed per .phtml file.

Requirements

1. Mapping file should be named baler-preload-map.json, and should be written to the root of a locale dir, as a sibling of requirejs-baler-config.js
2. A JSON Schema should be provided for Magento_Baler to use for validation
3. Key/Value should be
   • Key: Path from store root to a .phtml file
   • Value: Array of relative paths to JavaScript files from the require basedir

Example

{
   "vendor/magento/module-example/view/frontend/templates/some-template.phtml": ["path/to/file/1.js", "path/to/file/2.js"],
   "vendor/thirdparty/module-bar/view/frontend/templates/another-template.phtml": ["another/file.js", "yet/another/file/2.js"]
}